Ch. 10, The C++ string Class

Transcription

1 Ch. 10, The C++ string Class 1

2 Constructing a String You can create an empty string using string s no-arg constructor like this one: string newstring; You can create a string object from a string value or from an array of characters. To create a string from a string literal, use a syntax like this one: string newstring(stringliteral); 2

3 Appending a String You can use several overloaded functions to add new contents to a string. For example, see the following code: string s1("welcome"); s1.append(" to C++"); // appends " to C++" to s1 cout << s1 << endl; // s1 now becomes Welcome to C++ string s2("welcome"); s2.append(" to C and C++", 0, 5); // appends " to C" to s2 cout << s2 << endl; // s2 now becomes Welcome to C string s3("welcome"); s3.append(" to C and C++", 5); // appends " to C" to s3 cout << s3 << endl; // s3 now becomes Welcome to C string s4("welcome"); s4.append(4, 'G'); // appends "GGGG" to s4 cout << s4 << endl; // s4 now becomes WelcomeGGGG 3

4 Assigning a String You can use several overloaded functions to assign new contents to a string. For example, see the following code: string s1("welcome"); s1.assign("dallas"); // assigns "Dallas" to s1 cout << s1 << endl; // s1 now becomes Dallas string s2("welcome"); s2.assign("dallas, Texas", 0, 5); // assigns "Dalla" to s2 cout << s2 << endl; // s2 now becomes Dalla string s3("welcome"); s3.assign("dallas, Texas", 5); // assigns "Dalla" to s3 cout << s3 << endl; // s3 now becomes Dalla string s4("welcome"); s4.assign(4, 'G'); // assigns "GGGG" to s4 cout << s4 << endl; // s4 now becomes GGGG 4

5 Functions at, clear, erase, and empty You can use the at(index) function to retrieve a character at a specified index, clear() to clear the string, erase(index, n) to delete part of the string, and empty() to test if a string is empty. For example, see the following code: string s1("welcome"); cout << s1.at(3) << endl; // s1.at(3) returns c cout << s1.erase(2, 3) << endl; // s1 is now Weme s1.clear(); // s1 is now empty cout << s1.empty() << endl; // s1.empty returns 1 (means true) 5

6 Comparing Strings Often, in a program, you need to compare the contents of two strings. You can use the compare function. This function works in the same way as the C-string strcmp function and returns a value greater than 0, 0, or less than 0. For example, see the following code: string s1("welcome"); string s2("welcomg"); cout << s1.compare(s2) << endl; // returns -2 cout << s2.compare(s1) << endl; // returns 2 cout << s1.compare("welcome") << endl; // returns 0 6

7 Obtaining Substrings You can obtain a single character from a string using the at function. You can also obtain a substring from a string using the substr function. For example, see the following code: string s1("welcome"); cout << s1.substr(0, 1) << endl; // returns W cout << s1.substr(3) << endl; // returns come cout << s1.substr(3, 3) << endl; // returns com 7

8 Searching in a String You can use the find function to search for a substring or a character in a string. For example, see the following code: string s1("welcome to HTML"); cout << s1.find("co") << endl; // returns 3 cout << s1.find("co", 6) << endl; // returns -1 cout << s1.find('o') << endl; // returns 4 cout << s1.find('o', 6) << endl; // returns 9 8

9 Inserting and Replacing Strings Here are the examples to use the insert and replace functions: string s1("welcome to HTML"); s1.insert(11, "C++ and "); cout << s1 << endl; // s1 becomes Welcome to C++ and HTML string s2("aa"); s2.insert(1, 4, 'B'); cout << s2 << endl; // s2 becomes to ABBBBA string s3("welcome to HTML"); s3.replace(11, 4, "C++"); cout << s3 << endl; // returns Welcome to C++ 9

10 String Operators 10

11 Reading Strings string city; cout << "Enter a city: "; cin >> city; // Read to array city cout << "You entered " << city << endl; string city; cout << "Enter a city: "; getline(cin, city, '\n'); // Same as getline(cin, city) cout << "You entered " << city << endl; 11

12 Passing Objects to Functions You can pass objects by value or by reference. PassObjectByValue PassObjectByReference Run Run 12

13 Checking Palindromes A string is a palindrome if it reads the same forward and backward. The words mom, dad, and noon, for example, are all palindromes. How do you write a program to check whether a string is a palindrome? CheckPalindrome Run 13